Friday, February 17, 2012

Sailing the Titan Seas

The Titan Mare Explorer (TiME) is a proposed mission to Saturn’s moon Titan which will explore one of the hydrocarbon seas located in the northern region of the moon. TiME will be powered by an advanced Stirling radioisotope generator (ASRG) which provides over 100 watts of electrical power from 500 watts of heat derived from the decay of 800 grams of radioactive plutonium-238. The ASRG has a mass of 28 kilograms and a nominal lifetime of 14 years. This will allow TiME to operate on Titan for a long period of time where solar power is impractical.

TiME is currently in its conceptual phase and if selected, its launch is schedule to take place in 2016, with arrival at Titan in 2023. TiME will make a direct entry into Titan’s atmosphere from its interplanetary trajectory and descend under a parachute before “splashing down” on Ligeia Mare - a hydrocarbon sea in the high northern latitudes of Titan. Ligeia Mare measures roughly 500 kilometres in diameter and it has a surface area of around 100,000 square kilometres. Assuming that all goes well, TiME will perform the first ever exploration of an ocean environment beyond Earth for a nominal duration of 6 Titan days (96 Earth days).

The science objectives of the TiME mission are:
1. Determine the chemistry of a Titan sea.
2. Determine the depth of a Titan sea.
3. Constrain marine processes on Titan.
4. Determine how the local meteorology over the sea varies on diurnal timescales.
5. Characterize the atmosphere above the sea.

Ligeia Mare was first identified from a synthetic aperture radar (SAR) image acquired by the Cassini spacecraft on February 2007. In the SAR image, Ligeia Mare appears to be very dark and this is consistent with a very smooth surface. Radiometry data also indicates a liquid ethane/methane composition for Ligeia Mare. A relatively deep bathymetry is expected for Ligeia Mare since it appears pitch-black in radar observations with no appreciable bottom echo. In comparison, a lake near the South Pole of Titan named Ontario Lacus shows an appreciable bottom echo in radar observations which suggests a depth shallower than 6 meters.

A paper by Ralph D.Lorenz, at al. (2012) entitled “Winds and tides of Ligeia Mare, with application to the drift of the proposed time TiME (Titan Mare Explorer) capsule” investigates how the winds and tides on Ligeia Mare can affect the drift of TiME on the sea’s surface. The proportion of liquid ethane to methane is not known for Ligeia Mare. However, the composition may be ethane-rich since methane evaporates more readily from the sea’s surface. The paper suggests that the composition of Ligeia Mare is expected to have a significant effect on the winds around the sea. A methane-rich composition should result in gentler winds as the evaporating methane absorbs more of the incoming solar energy. On the other hand, an ethane-rich composition should lead to stronger winds. Assuming an ethane-rich sea, the wind speeds at Ligeia Mare appear to rarely exceed 1 m/s and never exceed 1.5 m/s. Furthermore, the likely drift rate calculated for TiME is around one tenth of the wind speed.

Using an assumed bathymetry, the tides in Ligeia Mare is modelled to occur in the form of a slosh between the eastern and western lobes of the sea. The maximum tidal amplitude of approximately 0.6 meters is expected to occur at the far end of the eastern lobe of Ligeia Mare. With the assumed bathymetry, the maximum tidal current of around 1 cm/s can be found in the eastern lobe. This shows that for TiME, its motion on Ligeia Mare is likely to be dominated by wind drag rather than by tidal currents. The drift of a capsule floating on the surface of Ligeia Mare will ultimately depend on the drag forces being exerted by the air and liquid on the cross-sectional areas of the capsule above and below the “waterline” respectively. Nevertheless, the amount of drift and/or drift contributions can be augmented by changing the cross-sectional areas of the capsule above and below the “waterline” or by drag modulation using a small actuator.